On Saturday night four American destroyers were moving closer to Syria, armed with Tomahawk cruise missiles, which are capable of precision strikes.

Gen Sir Nick Houghton, the Chief of the Defence Staff, is to take part in a summit in Jordan tomorrow with his US, French, Turkish, Saudi Arabian and Qatari counterparts.

It follows the strongest indications to date from Washington that direct military intervention by the West was possible in the conflict.

Diplomats talked of a “change in the American posture” following the attack on the suburb of East Ghouta on Wednesday.

Mr Cameron’s officials were drafting the text of a resolution to put before the UN said to be modelled on one that offered Saddam Hussein, the late Iraq leader, “a final opportunity” to disarm in 2002.

The move risks a public row with Vladimir Putin, the Russian president, who does not want any action taken against his ally.

But US officials were studying the Kosovo conflict, in which Nato launched weeks of air strikes without UN support and in the teeth of Russian opposition.

The Prime Minister is also to hold a meeting of the National Security Council. However, senior military figures have said privately that the “window of opportunity” for a successful intervention in Syria has long been closed.

Three Syrian hospitals yesterday told the humanitarian group Médecins Sans Frontières that they had received around 3,600 patients suffering from symptoms related to the attack. Of these, 355 had reportedly died.

President Assad’s regime has denied that it has used chemical weapons, describing the claims as “absolutely baseless”.